The removal of heavy metal ions from waste solutions has received considerable study due to their toxic nature. Adsorption on solid substrates is one of the main techniques that has been studied. Substrates include inorganic substances such as alumina, ferric oxide and manganese oxide, and organic substrates such as various plants, e.g., algae and water hyacinth.
The control of ferric iron in metallurgical processing solutions is a major problem facing the industry, particularly in solutions obtained from biotreatment of iron-containing sulfide ores. In some instances, the concentration of ferric iron is 5-20 grams/liter. Use of conventional neutralization and/or precipitation techniques is inadequate and difficult to implement commercially.
The use of an acid phosphate salt, followed by pH adjustment and addition of a calcium source has been disclosed for making toxic metals such as lead, chromium, cadmium, arsenic, selenium, silver and barium less soluble in waste sludges. See U.S. Pat. No. 4,671,882 issued Jun. 9, 1987 to Douglas at al. for "Phosphoric Acid/Lime Hazardous Waste Detoxification Treatment Process." The use of phosphate materials has been disclosed for rendering lead and cadmium in solid wastes even more insoluble. See U.S. Pat. No. 4,737,356 issued Apr. 12, 1988 to O'Hara et al. for "Immobilization of Lead and Cadmium in Solid Residues from the Combustion of Refuse Using Lime and Phosphate." The use of solid calcium phosphate materials, e.g. gypsum powder, has been disclosed to fixate and stabilize leachable lead in toxic wastes. See U.S. Pat. No. 5,193,936 issued Mar. 16, 1993 to Pal et al. for "Fixation and Stabilization of Lead in Contaminated Soil and Solid Waste." Phosphorylated polysaccharides, chitin phosphate and chitosan phosphate have been disclosed as useful to adsorb heavy metal ions such as uranium. See Sakaguchi, T., et al. (1993), "Recovery and Removal of Heavy Metal Elements such as Uranium by using Phosphate Compounds," (1933) in Beneficiation of Phosphate: Theory and Practice, H. El-Shall, et al., eds., Society for Mining, Metallurgy and Exploration, Littleton, Colo., Chapter 44, p. 463.
The use of hydroxyapatite materials has been disclosed for removal of certain heavy metal ions from solution. For example, Suzuki, T., et al. (1980), "Synthetic Hydroxyapatites Employed as Inorganic Cation-exchangers," (1981) J. Chem. Soc. Faraday Trans. 77:1059-1062 discloses that ions such as Cd.sup.2+, Zn.sup.2+, Ni.sup.2+, Mg.sup.2+, and Ba.sup.2+ are removed from aqueous solution by a mechanism involving ion exchange using synthetic hydroxyapatite. Lead (Pb.sup.2+) was shown to be exchanged for Ca.sup.2+ ions in aqueous solutions using synthetic hydroxyapatites. See Suzuki, T., et al. (1984), "Synthetic Hydroxyapatites as Inorganic Cation Exchanges," J. Chem. Soc. Farady Trans. 80:3157-3165; and Takeuchi, Y., et al. (1988), "Study of Equilibrium and Mass Transfer in Processes for Removal of Heavy-metal Ions by Hydroxyapatite," (1988) J. Chem Eng. of Japan 21:98-100. Solid calcium phosphate materials such as naturally-occurring apatite and synthetic hydroxyapatite have been disclosed as useful for in-situ immobilization of lead-contaminated soils, wastes and sediments by mixing with the lead-contaminated materials and leaving the mixture in place. See U.S. Pat. No. 5,512,702 issued Apr. 30, 1996 to Ryan, et al. for "Method for In-Situ Immobilization of Lead in Contaminated Silts, Wastes, and Sediments Using Solid Calcium Phosphate Materials." Hydroxyapatite has been disclosed as a useful component of filtering material for drinking water for removal of lead. See U.S. Pat. No. 5,665,240 issued Sep. 9, 1997 to Hong for "Point-of-Use Removal of Lead in Drinking Water Using Phosphate and Carbonate Minerals." Use of hydroxyapatite or a calcium depleted hydroxyapatite for immobilization of heavy metals in toxic waste materials has been disclosed. See U.S. Pat. No. 5,678,233 issued Oct. 14, 1997 to Brown for "Method of Immobilizing Toxic or Radioactive Inorganic Wastes and Associated Products." Hydroxyapatite has also been disclosed as useful or the removal of heavy metals from aqueous brine. See U.S. Pat. No. 5,681,447 issued Oct. 28, 1997 to Maycock, et al. for "Removal of Trace Metal and Metalloid Species from Brine."
Although hydroxyapatite materials have been known to be able to remove certain heavy metal ions from solution, their efficiency in doing so in view of their cost has precluded their use in many applications. A process using a less expensive, more efficient material is therefore needed.